Process for recovering SF6 from a gas

ABSTRACT

A process for recovering SF 6  from a gas is provided. The process includes the step of contacting a gas stream comprising SF 6  and at least one of N 2 , O 2 , CO 2 , and H 2  O with a membrane in at least one membrane separation unit at conditions effective to obtain a retentate stream rich in SF 6  and a permeate stream rich in at least one of N 2 , O 2 , CO 2 , and H 2  O.

FIELD OF THE INVENTION

The present invention generally relates to a gas separation process. The invention particularly relates to a process for recovering SF₆ and/or CO₂ from a gas stream using membrane separation techniques. The invention also particularly relates to a process for forming solidified magnesium metal, which process includes recovering and/or recycling of SF₆ and/or CO₂.

BACKGROUND OF THE INVENTION

Magnesium is often formed into ingots, bars, sheets, or rods, and sold as a commercial product. Typically, the magnesium is given a particular shape by a casting process which involves melting the magnesium and pouring it into a mold.

Magnesium in the molten state, however, requires some form of protection to inhibit oxidation and to prevent burning. When molten magnesium is exposed to ambient air, a thick layer of magnesium oxide forms on the surface of the melt. However, this layer of magnesium oxide is not adequate to provide the necessary protection. In a matter of just seconds, many blooms form on the surface of the molten magnesium which first glow red and then burst into white flames. As a result, the MgO (on the surface) is not smooth or coherent with the molten surface. Moreover, MgO occupies much less volume than the metal from which it is formed. Thus, the entire surface of the molten magnesium that is exposed to ambient air is not protected. In addition, the boiling point of magnesium is relatively low. This results in a high vapor pressure which further puts critical demands on the MgO layer.

One solution to the oxidation problem has been to isolate the melt from the air with a non-reactive barrier consisting of a flux of salts (e.g., 50% MgCl₂, 25% KCl, 18% BaCl₂, 4% CaF₂, 3% MgO). The flux is liquid at the temperature of molten magnesium and has a surface tension that enables the flux to encase the melt with a relatively impermeable fluid film.

However, the use of a salt flux cover has a number of drawbacks. For example, it reduces the magnesium metal yield. The salt flux forms a sludge composed of oxides and inter-metallic particles. The sludge is generated by oxidation and agitation of the molten metal bath. Oxidation of the salt flux also produces dross floating on the metal surface. Both the sludge and the dross have a considerable amount of metallic magnesium trapped in their structure which reduces yield.

The sludge and dross also create other problems. For example, the sludge settles to the bottom of the pot where it may insulate and cause hot spots on the pot. Additionally, both present disposal problems. Flux can contaminate (e.g., corrosion effects) the finished castings by forming flux inclusions or oxides. Flux fumes and flux dust can also cause serious corrosion problems in the plant.

Another approach to solving the problem associated with molten magnesium is to use inhibitors in the air. The early practice was to burn coke or sulfur to produce the gaseous agent, CO₂ or SO₂. An atmosphere of CO₂ was found to be superior to the commonly used commercial atmospheres of N₂, Ar, or He because of the absence of vaporization of the magnesium, the absence of excessive reaction products, and the necessity for the enclosure above the molten metal to be extremely air tight. Some of these gases are also very expensive to use. Additionally, small amounts of SO₂ (<1%) was found to be sufficient for protecting the molten magnesium from burning, even if air and moisture are present in the atmosphere above the melt.

However, the use of these inhibitors also has several drawbacks. For example, both CO₂ and SO₂ pose pollution problems such as breathing discomfort for plant personnel, residual sludge disposal, and a corrosive atmosphere which is detrimental to both plant and equipment. Furthermore, SO₂ is toxic and can cause explosions.

While BF₃ has been mentioned as being a very effective inhibitor, it is not suitable for commercial processes because it is extremely toxic and corrosive.

Sulfur hexafluoride (SF₆) has also been mentioned as one of many fluorine-containing compounds that can be used in air as an oxidation inhibitor for molten magnesium. However, it was overlooked for many years. When its important dielectric properties were applied to electric equipment, it was tried as an inhibitor in die casting operations. SF₆ was discarded at that point, however, because of its severe attack on the ferrous equipment. In addition, the use of pure SF₆ for protecting molten magnesium has been reported to have caused explosions.

Later, it was found that at low concentrations of SF₆ in air (<1%), a protective thin film of MgO (and MgF₂) is formed on the magnesium melt surface. Advantageously, even at high temperatures in air, SF₆ showed negligible or no reactions.

However, the combination of SF₆ and air has some drawbacks. For example, since SF₆ is adsorbed by the thin film, there must be a continuous supply of the gas to maintain the otherwise unstable film. Moreover, at higher temperatures, higher concentrations of SF₆ are necessary. At the higher concentrations, SO₂ and MgF₂ are formed, and magnesium is wastefully further oxidized.

It was then found that CO₂ could be used together with SF₆ and air. A gas atmosphere of air, SF₆, and CO₂ has several advantages. First, it is non-toxic and non-corrosive. Second, it eliminates the use of salt fluxes and the need to dispose of the resulting sludge. Third, using such a gas atmosphere results in a lower metal loss, elimination of corrosion effects, and clean castings. Fourth, such a casting process provides a clean operation and improves the working conditions. In short, the addition of CO₂ provides much improved protection compared to the melt protection obtained with air/SF₆ mixtures.

However, using a gas atmosphere of SF₆ and CO₂ also has some disadvantages. Namely, both SF₆ and CO₂ have been mentioned as being greenhouse gases, i.e., they have a high global warming potential. In addition, SF₆ is a relatively expensive gas. Thus, there is a need in the art to reduce the total amount of SF₆ and CO₂ that is used and released into the atmosphere.

Accordingly, it is an object of the present invention to address this need in the magnesium industry. In particular, it is an object of the present invention to provide an efficient and economical process for recovering SF₆ and/or CO₂ from the vent gas of a magnesium foundry and optionally recycling the recovered gas. These and other objects of the invention will become apparent in light of the following specification, and the appended drawings and claims.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a process for recovering SF₆ from a gas. The process includes the step of contacting a gas stream comprising SF₆ and at least one of N₂, O₂, CO₂, and H₂ O with a membrane in at least one membrane separation unit at conditions effective to obtain a retentate stream rich in SF₆ and a permeate stream rich in at least one of N₂, O₂, CO₂, and H₂ O.

In one embodiment, the process according to the present invention involves separating SF₆ from CO₂. The process includes the step of contacting a gas stream comprising SF₆ and CO₂ with a membrane in two or more membrane separation units at conditions effective to obtain a retentate stream rich in SF₆ and a permeate stream rich in CO₂.

Preferably, the gas stream is contacted with a membrane in three membrane separation units. The gas stream is contacted with a first membrane in a first membrane separation unit at conditions effective to obtain a first retentate stream rich in SF₆ and a first permeate stream rich in CO₂. The first permeate stream is then contacted with a second membrane in a second membrane separation unit at conditions effective to obtain a second retentate stream depleted in CO₂ and a second permeate stream rich in CO₂. The first retentate stream is contacted with a third membrane in a third membrane separation unit at conditions effective to obtain a third retentate stream rich in SF₆ and a third permeate stream depleted in SF₆.

In another aspect, the present invention relates to a process for forming solidified Mg metal. The process includes the steps of:

(a) casting molten Mg metal in a gaseous atmosphere comprising SF₆ ;

(b) cooling the molten Mg metal to form a solidified Mg metal;

(c) obtaining a vent gas comprising SF₆ and at least one of N₂, O₂, CO₂, and H₂ O from the casting step; and

(d) contacting the vent gas with a membrane in at least one membrane separation unit at conditions effective to obtain a retentate stream rich in SF₆ and a permeate stream rich in at least one of N₂, O₂, CO₂, and H₂ O. Preferably, at least a portion of the retentate stream is recycled to the casting step to provide the gaseous atmosphere.

In one embodiment, the process for forming solidified Mg metal according to the present invention includes the steps of:

(a) casting molten Mg metal in a gaseous atmosphere comprising SF₆ and CO₂ ;

(b) cooling the molten Mg metal to form a solidified Mg metal;

(c) obtaining a vent gas comprising SF₆ and CO₂ from the casting step; and

(d) contacting the vent gas with a membrane in two or more membrane separation units at conditions effective to obtain a retentate stream rich in SF₆ and a permeate stream rich in CO₂. Preferably, at least a portion of the retentate stream and at least a portion of the permeate stream are recycled to the casting step to provide the gaseous atmosphere.

In a particularly preferred embodiment, prior to being recycled, the permeate stream and the retentate stream are contacted with a second and third membrane, respectively, to improve the purity of those streams.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in greater detail with reference to the accompanying drawings in which like elements bear like reference numerals, and wherein:

FIG. 1 illustrates an engineering design of a membrane system that can be employed in the present invention;

FIG. 2 is a flow diagram of a preferred process for recovering SF₆ and CO₂ according to the present invention;

FIG. 3 is a flow diagram of a preferred process for forming solidified Mg metal according to the present invention;

FIG. 3A is a flow diagram of another preferred process for forming solidified Mg metal according to the present invention;

FIG. 4 is a flow diagram of a preferred process for forming solidified Mg metal according to the present invention which incorporates the process for recovering SF₆ and CO₂ depicted in FIG. 2; and

FIG. 5 is a graph showing the permeability of N₂ and SF₆ through a particular membrane at different partial pressures of SF₆.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a first aspect, the present invention relates to a process for recovering SF₆ from a gas. The process includes the step of contacting a gas stream comprising SF₆ and at least one of N₂, O₂, CO₂, and H₂ O with a membrane in at least one membrane separation unit at conditions effective to obtain a retentate stream rich in SF₆ and a permeate stream rich in at least one of N₂, O₂, CO₂, and H₂ O.

As used in this specification and the claims, the term "rich" means that the concentration of a particular component in that stream is greater than the concentration of the same component in the feed stream to the particular process step. Likewise, the term "depleted" means that the concentration of a particular component in that stream is less than the concentration of the same component in the feed stream to the particular process step.

Preferably, the gas stream to be separated contains from about 0.0001 to about 20% by volume of SF₆, and the retentate stream contains from about 0.01 to about 100% by volume of SF₆.

In a preferred embodiment, the purity of the permeate and retentate streams is improved by contacting the streams with additional membrane separation units arranged in series. Such a process may be carried out according to the multi-step membrane separation system shown in FIG. 1. As shown in FIG. 1, each of the permeate and retentate streams may be passed to another membrane contacting unit to increase the purity of the stream. The number of contacting steps "m" and "n" may vary depending on the purity desired. By using such a process, it is possible to obtain 100% recovery of SF₆ with a purity approaching 100% by volume.

Any membrane can be used in the process of the present invention so long as the membrane can selectively retain SF₆ while passing the other components in the gas stream through. The membrane should also be substantially non-reactive with the gaseous components to be separated.

Membranes suitable for use in the present invention include glassy membranes such as polymer membranes made preferably from polyimides; polyamides; polyamide-imides; polyesters polycarbonates; polysulfones; polyethersulfone; polyetherketone; alkyl substituted aromatic polyesters; blends of polyethersulfone, aromatic polyimides, aromatic polyamides, polyamides-imides, fluorinated aromatic polyimide, polyamide, and polyamide-imides; glassy polymeric membranes such as those disclosed in U.S. Ser. No. 08/247,125 filed May 20, 1994, the content of which is hereby incorporated by reference; cellulose acetates; and blends thereof, copolymers thereof, substituted polymers (e.g. alkyl, aryl) thereof and the like.

Other membranes suitable for use in the present invention include asymmetric membranes. Asymmetric membranes are prepared by the precipitation of polymer solutions in solvent-miscible nonsolvents. Such membranes are typified by a dense separating layer supported on an anisotropic substrate of a graded porosity and are generally prepared in one step. Examples of such membranes and their methods of manufacture are disclosed in U.S. Pat. Nos. 4,113,628; 4,378,324; 4,460,526; 4,474,662; 4,485,056; 4,512,893; 5,085,676; and 4,717,394; all incorporated herein by reference. The '394 and '676 patents disclose preparation of asymmetric separation membranes from selected polyimides. Particularly preferred membranes are polyimide asymmetric gas separation membranes as disclosed in the '676 patent.

Yet other membranes suitable for use in the present invention include composite gas separation membranes. These membranes typically have a dense separating layer on a preformed microporous substrate. The separating layer and the substrate are usually different in composition. Composite gas separation membranes have evolved to a structure of an ultrathin, dense separating layer supported on an anisotropic, microporous substrate. These composite membrane structures can be prepared by laminating a preformed ultrathin dense separating layer on top of a preformed anisotropic support membrane. Examples of such membranes and their methods of manufacture are disclosed in U.S. Pat. Nos. 4,664,669; 4,689,267; 4,741,829; 2,947,687; 2,953,502; 3,616,607; 4,714,481; 4,602,922; 2,970,106; 2,960,462; 4,713,292; 4,086,310; 4,132,824; 4,192,824; 4,155,793; and 4,156,597; all incorporated herein by reference.

Alternatively, composite gas separation membranes may be prepared by multistep fabrication processes, wherein first an anisotropic, porous substrate is formed, followed by contacting the substrate with a membrane-forming solution. Examples of such methods are described in U.S. Pat. Nos. 4,826,599; 3,648,845; and 3,508,994; all incorporated herein by reference.

U.S. Pat. No. 4,756,932 describes how composite hollow-fiber membranes may also be prepared by co-extrusion of multiple polymer solution layers, followed by precipitation in a solvent-miscible nonsolvent.

In the case of hollow-fiber membranes, the term "membrane" as used in the present invention is intended to include both the individual hollow fibers as well as multiple hollow fibers.

The membrane used in the present invention can be post-treated with, or coated by, or co-extruded with, a fluorinated or perfluorinated polymer layer in order to increase its ability to withstand harmful constituents in the gas stream from which SF₆ is to be separated, at low levels or temporary contact with such components.

The temperature of the gas stream and/or the membrane during the contacting step in each membrane separation unit can vary from about -10° C. to about 100° C. Preferably, the temperature is between about 10° C. and 80° C. More preferably, the temperature ranges from ambient, i.e., from about 20° C. to 25° C., to about 60° C.

It is preferred, according to the present invention, to have a pressure drop across the membrane in each membrane separation unit of less than about 2,000 psig. More preferably, the pressure drop ranges from about 3 to about 200 psig. Even more preferably, the pressure drop is about 20 to about 60 psig.

The requisite pressure drop across the membrane can be provided in one of two ways. First, the feed gas stream can be compressed. Preferred compressors are sealed and oil-free, such as the compressors sold under the tradename POWEREX, available from the Powerex Harrison Company of Ohio. Second and more preferably, the pressure drop across the membrane can be established by lowering the pressure on the permeate side of the membrane. To create the lower pressure on the permeate side, a vacuum pump or any other suction device can be used.

The flowrate of the gas stream across the membrane in each membrane separation unit can vary from about 0 to about 10⁵ Nm³ /h per square meter of membrane available for separation. Preferably, the flowrate ranges from about 10⁻⁴ to about 10 Nm³ /h-m². More preferably, the flowrate ranges from about 0.1 to about 0.5 Nm³ /h-m².

Of course, the particular contacting conditions in each membrane separation unit may be the same or different, depending on various factors including the type of membrane employed as well as the degree of separation or purity desired. The selection of such parameters is within the level of skill of the ordinary worker in this art.

In another preferred embodiment, the process according to the present invention is used to separate SF₆ from CO₂ and that two or more membrane separation units are employed. Preferably, the gas stream to be separated comprises from about 0.0001 to about 20% by volume of SF₆ and from about 0.01 to about 50% by volume of CO₂. Also preferably, the gas stream to be separated further comprises at least one of N₂, O₂, and H₂ O. More preferably, the gas stream comprises both N₂ and O₂. An example of such an embodiment of the present invention is shown in FIG. 2.

Referring to FIG. 2, a gas stream 10 comprising SF₆, CO₂, N₂, and O₂ is contacted with a first membrane separation unit 100 at conditions effective to obtain a first retentate stream 11 rich in SF₆ and a first permeate stream 13 rich in CO₂. The first retentate stream preferably comprises from about 0.01 to about 100% by volume of SF₆, and the first permeate stream preferably comprises from about 0.1 to about 90% by volume of CO₂.

The first permeate stream 13 is then contacted with a second membrane separation unit 110 at conditions effective to obtain a second retentate stream 14 depleted in CO₂ and a second permeate stream 15 rich in CO₂. The first retentate stream 11 is contacted with a third membrane separation unit 120 at conditions effective to obtain a third retentate stream 12 rich in SF₆ and a third permeate stream 16 depleted in SF₆. In addition to being depleted in SF₆ and CO₂, the second retentate stream 14 and the third permeate stream 16 are rich in at least one of N₂ and O₂.

The particular membrane used in each membrane separation unit 100, 110, and 120 may be the same or different. The membranes are selected depending on various considerations known to those skilled in the art such as the degree of separation desired.

In a second aspect, the present invention relates to a process for forming solidified Mg metal. The process includes the steps of:

(a) casting molten Mg metal in a gaseous atmosphere comprising SF₆ ;

(b) cooling the molten Mg metal to form a solidified Mg metal;

(c) obtaining a vent gas comprising SF₆ and at least one of N₂, O₂, CO₂, and H₂ O from the casting step; and

(d) contacting the vent gas with a membrane in at least one membrane separation unit at conditions effective to obtain a retentate stream rich in SF₆ and a permeate stream rich in at least one of N₂, O₂, CO₂, and H₂ O. Preferably, at least a portion of the retentate stream is recycled to the casting step to provide at least part of the gaseous atmosphere. Prior to being recycled, the retentate stream can be mixed with at least one of N₂, O₂, make-up SF₆ and CO₂.

A preferred exemplary embodiment of this process is shown in FIG. 3. Referring to FIG. 3, a stream 20 of molten Mg metal is fed into a casting chamber 130 having a gaseous atmosphere comprising SF₆. The molten Mg metal is then cooled to yield a solidified Mg metal which is withdrawn in line 21. The operating conditions and equipment necessary to carry out this step are well known to those skilled in the art.

The gaseous atmosphere in the casting chamber 130 preferably comprises from about 0.01 to about 20% by volume of SF₆, and more preferably from about 0.1 to about 10% by volume of SF₆. The balance of the gaseous atmosphere can be a mixture of N₂ and O₂ such as air, as well as other gaseous components including CO₂ and H₂ O.

A vent gas 22 is withdrawn from the casting chamber 130. The vent gas 22 comprises SF₆, N₂, O₂, and H₂ O, and optionally HCl, Cl₂ and particulates. The vent gas 22 typically contains from about 0.0001 to about 0.2% by volume of SF₆. If HCl, Cl₂, and particulates are present, then the vent gas 22 can be passed to a pretreatment unit 140 to remove these components.

Preferably, as shown in FIG. 3A, the pretreatment unit 140 comprises filters 141 and 144, and a dry scrubber 142 and a wet scrubber 143. The vent gas 22 is passed into filter 141 to remove the particulates. The gas stream 22a exiting the filter 141 is introduced into the dry scrubber 142 to remove some of the halide components. The gas stream 22b exiting the dry scrubber 142 is then transferred to the wet scrubber 143 wherein the remaining halides are removed. The wet scrubber 143 produces a gas stream 22c which is essentially free of the halides. The gas stream 22c is then passed to an additional filter 144 to remove any remaining particulates in the gas stream 22c.

Of course, the particular type of filters and scrubbers employed can be readily determined by those skilled in the art. Also, the number and the relative positions of the filters and scrubbers may vary depending on the desired purity of the resulting gas stream.

By way of example, dry scrubbers are usually resin-type scrubbers or soda-lime. Exemplary wet scrubbers that can be used in the present invention are described in the brochure entitled, "Selecting a CDO™ for Your Particular Application" from DELATECH Corporation, which brochure is hereby incorporated by reference. When various harmful constituents, e.g., halides, have to be removed, it is preferred to use a dry scrubber or scrubbers in series with a wet scrubber or scrubbers.

Preferably, upstream of the scrubber or scrubbers, one or more filters are employed to remove the particulates from the vent gas. It is preferred to use a filter having a pore size diameter of less than 20 micrometers, and more preferably, less than 10 micrometers.

If there are remaining particulates in the purified vent gas before it is passed to the membrane separation unit, it is contemplated by the present invention to employ an additional filter or filters to remove such particulates.

Referring again to FIG. 3, a gas stream 23 comprising SF₆, N₂, O₂, and H₂ O is withdrawn from the pretreatment unit 140. The gas stream 23 is then passed to a membrane separation unit 150 to produce a retentate stream 25 rich in SF₆ and a permeate stream 24 depleted in SF₆. The permeate stream 24, which primarily contains N₂, O₂, and H₂ O, can be vented to the atmosphere.

The concentration of SF₆ in the retentate stream 25 can be set at a desired level by changing the parameters of the membrane separation unit 150 (e.g., pressure drop across the membrane, temperature, and feed to membrane area ratio). Preferably, the retentate stream 25 comprises from about 30 to about 100% by volume of SF₆.

The SF₆ concentration of the retentate stream 25 can also be adjusted in a mixing unit 160 wherein the recovered SF₆ is mixed with a mixture of N₂ and O₂ 26, and/or makeup SF₆ 27 to yield a recycle stream 28. It should be noted that the ratio of N₂ /O₂ added to the recycle stream 28 can be adjusted so that the recycle stream 28 can have the desired ratio of the gas constituents. The recycle stream 28 is then passed to the casting chamber 130 to provide at least part of the SF₆ gaseous atmosphere.

In another preferred embodiment, the process for forming solidified Mg metal according to the present invention includes the steps of:

(a) casting molten Mg metal in a gaseous atmosphere comprising SF₆ and CO₂ ;

(b) cooling the molten Mg metal to form a solidified Mg metal;

(c) obtaining a vent gas comprising SF₆ and CO₂ from the casting step; and

(d) contacting the vent gas with a membrane in two or more membrane separation units at conditions effective to obtain a retentate stream rich in SF₆ and a permeate stream rich in CO₂. Preferably, at least a portion of the retentate stream and at least a portion of the permeate stream are recycled to the casting step to provide the gaseous atmosphere. More preferably, prior to being recycled, the permeate stream and the retentate stream are contacted with a second and third membrane, respectively, to improve the purity of those streams.

A particularly preferred exemplary embodiment of this process is shown in FIG. 4. Referring to FIG. 4, a stream 20 of molten Mg metal is fed into a casting chamber 130 having a gaseous atmosphere comprising SF₆ and CO₂. The molten Mg metal is then cooled to yield a solidified Mg metal which is withdrawn in line 21.

The gaseous atmosphere in the casting chamber 130 preferably comprises from about 0.01 to about 20% by volume of SF₆ and from about 30 to about 70% by volume of CO₂. More preferably, the gaseous atmosphere comprises from about 0.1 to about 10% by volume of SF₆ and from about 40 to about 60% by volume of CO₂. The balance of the gaseous atmosphere can be a mixture of N₂ and O₂ such as air, as well as other gaseous components including H₂ O.

A vent gas 22 is withdrawn from the casting chamber 130. The vent gas 22 comprises SF₆, CO₂, N₂, and O₂ and optionally HCl, Cl₂, and particulates. The vent gas 22 typically contains about 0.0001 to about 0.2% by volume of SF₆ and from about 2 to about 30% by volume of CO₂. If HCl, Cl₂, and particulates are present, then the vent gas 22 can be passed to a pretreatment unit 140 to remove those components.

A gas stream 23 comprising SF₆, CO₂, N₂, and O₂ is then withdrawn from the pretreatment unit 140. The gas stream 23 is passed to a membrane separation system 150 that resembles the configuration depicted in FIG. 2.

Again referring to FIG. 4, the gas stream 23 is contacted with a first membrane separation unit 151 at conditions effective to obtain a first retentate stream 23a rich in SF₆ and a first permeate stream 23b rich in CO₂. The first retentate stream 23a preferably comprises from about 0.01 to about 100% by volume of SF₆, and the first permeate stream 23b preferably comprises from about 0.1 to about 90% by volume of CO₂.

The first permeate stream 23b is then contacted with a second membrane separation unit 152 at conditions effective to obtain a second retentate stream 23c depleted in CO₂ and a second permeate stream 25a rich in CO₂. The first retentate stream 23a is contacted with a third membrane separation unit 153 at conditions effective to obtain a third retentate stream 25 rich in SF₆ and a third permeate stream 23d depleted in SF₆.

In addition to being depleted in SF₆ and CO₂, the second retentate stream 23c and the third permeate stream 23d are rich in at least one of N₂ and O₂. These streams can been combined to form a purified vent stream 24.

The second permeate stream 25a and the third retentate stream 25 can be mixed together in a conditioner unit 170 along with a mixture of N₂ and O₂ 26, and make-up CO₂ 26a and make-up SF₆ 27 to form a recycle stream 28. Again, the ratio of N₂ /O₂ can be adjusted to obtain any desired concentration in the recycle stream 28. The recycle stream 28 is passed to the casting chamber 130 to provide at least part of the N₂ /O₂ /SF₆ /CO₂ gaseous atmosphere.

EXAMPLES

The following examples are provided to illustrate the present invention and are not to be construed as a limitation thereof.

EXAMPLE 1

A gas stream comprising SF₆ and N₂ was contacted with an asymmetric composite hollow fiber membrane with a surface area of 0.13 m² at various partial pressures of SF₆ to determine its permeability relative to N₂. The gas stream was contacted with the membrane at a temperature of 20° C. and at a flowrate rate of 200 sccm. The pressure of the feed gas was kept at 3 bar. The permeability of each of these components is graphically shown in FIG. 5. As seen from FIG. 5, the selectivity of SF₆ /N₂ is on the order of 500.

Based on the above selectivity, a computer simulation of a single stage membrane separation unit was conducted. The concentration and the pressure of the feed, permeate, and retentate as well as the percentage recovery of SF₆ are shown in Table 1 below.

                  TABLE 1     ______________________________________                                       SF.sub.6              Feed Permeate   Retentate                                       Recovery     ______________________________________     Flowrate   0.19   0.15       0.04     (Nm.sup.3 /h-m.sup.2)      SF.sub.6 !                0.01   9.2 × 10.sup.-5                                  1     (vol %)      N.sub.2 ! 99.99  99.99991   0     (vol %)     Pressure   9      1          8.7     (bar)                                         99.7%     ______________________________________

EXAMPLE 2

In an actual experiment, a gas stream comprising 0.1% by volume of SF₆ and the balance air was contacted with an asymmetric composite hollow fiber membrane with a surface area of 690 m² at a temperature of 20° C. in a cascade design process as shown in FIG. 1 wherein n=1 and m=0. The flowrate, concentration, and pressure of the feed, permeate, and retentate streams are reported below in Table 2. The percentage recovery of SF₆ is also reported in Table 2.

                  TABLE 2     ______________________________________                                       SF.sub.6              Feed Permeate   Retentate                                       Recovery     ______________________________________     Flowrate   447.7  443.62     4.08     (scfh)      SF.sub.6 !                0.100  0.0075     10.0670     (vol %)     Pressure   5.6    1          5.5     (bar)                                         92.3%     ______________________________________

EXAMPLE 3

In a computer simulation, a gas stream comprising SF₆, CO₂, and air is passed to a three-stage membrane separation system as shown in FIG. 2 to recovery both SF₆ and CO₂. The membrane in each stage is the same asymmetric composite hollow fiber membrane and the contacting temperature in each stage is 20° C. The flowrate, concentration, and pressure of the feed, permeate, and retentate streams of each stage are reported below in Table 3. The percentage recovery of SF₆ and CO₂ is also reported in Table 3.

                  TABLE 3     ______________________________________             Feed   Permeate  Retentate Recovery     ______________________________________     First Stage     Flowrate  0.25     0.21      0.04     (Nm.sup.3 /h-m.sup.2)      SF.sub.6 ! (vol %)               0.02     0.00      0.12    100%      CO.sub.2 ! (vol %)               5.00     6.00      9.00    100%      N.sub.2 ! (vol %)               75.98    71.19     99.86      O.sub.2 ! (vol %)               19.00    22.81     0.02     Pressure (bar)               9        0.5       8.7     Second Stage     Flowrate  0.63     0.23      0.40     (Nm.sup.3 /h-m.sup.2)      SF.sub.6 ! (vol %)               0.00     0.00      0.00      CO.sub.2 ! (vol %)               6.00     16.68     0.07       99.3%      N.sub.2 ! (vol %)               71.19    46.11     85.11      O.sub.2 ! (vol %)               22.81    37.21     14.82     Pressure (bar)               8        0.9       7.7     Third Stage     Flowrate  0.1270   0.1266    0.0004     (Nm.sup.3 /h-m.sup.2)      SF.sub.6 ! (vol %)               0.12     0.00      34.96    99%      CO.sub.2 ! (vol %)               0.00     0.00      0.00      N.sub.2 ! (vol %)               99.86    99.98     65.04      O.sub.2 ! (vol %)               0.02     0.02      0.00     Pressure (bar)               6.98     0.2       6.68     ______________________________________

While the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and the scope of the claims appended hereto. 

What is claimed is:
 1. A process for forming solidified Mg metal, said process comprising the steps of:(a) casting molten Mg metal in a gaseous atmosphere comprising SF₆ ; (b) cooling said molten Mg metal to form a solidified Mg metal; (c) obtaining a vent gas comprising SF₆ and at least one of N₂, O₂, CO₂, and H₂ O from said casting step; and (d) contacting said vent gas with a membrane in at least one membrane separation unit at conditions effective to obtain a retentate stream rich in SF₆ and a permeate stream rich in at least one of N₂, O₂, CO₂, and H₂ O.
 2. The process according to claim 1, wherein said gaseous atmosphere comprises from about 0.1 to about 10% by volume of SF₆.
 3. The process according to claim 1, wherein said vent gas comprises from about 0.0001 to about 0.2% by volume of SF₆.
 4. The process according to claim 1, wherein said retentate stream comprises from about 5 to about 100% by volume of SF₆.
 5. The process according to claim 1, wherein said conditions comprise a temperature between about 10° and about 80° C., a pressure drop between about 3 and about 200 psig, and a flowrate rate between about 10⁻⁴ and about 10 Nm³ /h-m².
 6. The process according to claim 1, wherein said membrane is selected from the group consisting of polyimides, polyamides, polyamide-imides, polyesters, polycarbonates, polysulfones, polyethersulfone, polyetherketone, alkyl substituted aromatic polyesters, and blends of polyethersulfone, aromatic polyimides, aromatic polyamides, polyamides-imides, fluorinated aromatic polyimide, polyamide, and polyamide-imides.
 7. The process according to claim 1, wherein said vent gas is contacted with said membrane in two or more membrane separation units.
 8. The process according to claim 1, wherein said vent gas additionally comprises at least one of HCl and Cl₂, and wherein said vent gas is contacted with at least one caustic scrubber to remove said at least one of HCl and Cl₂ from said vent gas prior to contacting with said membrane.
 9. The process according to claim 1, wherein said vent gas additionally comprises particulates, and wherein said vent, gas is contacted with at least one filter to remove said particulates from said vent gas prior to contacting with said membrane.
 10. The process according to claim 1, wherein at least a portion of said retentate stream is recycled to said casting step to provide said gaseous atmosphere.
 11. The process according to claim 10, wherein said retentate stream is mixed with a mixture of N₂ and O₂ prior to being recycled to said casting step.
 12. A process for forming solidified Mg metal, said process comprising the steps of:(a) casting molten Mg metal in a gaseous atmosphere comprising SF₆ and CO₂ ; (b) cooling said molten Mg metal to form a solidified Mg metal; (c) obtaining a vent gas comprising SF₆ and CO₂ from said casting step; and (d) contacting said vent gas with a membrane in two or more membrane separation units at conditions effective to obtain a retentate stream rich in SF₆ and a permeate stream rich in CO₂.
 13. The process according to claim 12, wherein said gaseous atmosphere comprises from about 0.1 to about 10% by volume of SF₆ from about 40 to about 60% by volume of CO₂, and the balance N₂ and O₂.
 14. The process according to claim 12, wherein said vent gas comprises from about 0.0001 to about 0.2% by volume of SF₆ and from about 2 to about 30% by volume of CO₂.
 15. The process according to claim 12, wherein said retentate stream comprises from about 0.01 to about 100% by volume of SF₆, and said permeate stream comprises from about 0.1 to about 90% by volume of CO₂.
 16. The process according to claim 12, wherein said conditions in each membrane separation unit comprise a temperature between about 10° and about 80° C., a pressure drop between about 3 and about 200 psig, and a flowrate rate between about 10⁻⁴ and about 10 Nm³ /h-m².
 17. The process according to claim 12, wherein said membrane is selected from the group consisting of polyimides, polyamides, polyamide-imides, polyesters, polycarbonates, polysulfones, polyethersulfone, polyetherketone, alkyl substituted aromatic polyesters, and blends of polyethersulfone, aromatic polyimides, aromatic polyamides, polyamides-imides, fluorinated aromatic polyimide, polyamide, and polyamide-imides.
 18. The process according to claim 12, wherein said vent gas additionally comprises at least one of HCl and Cl₂, and wherein said vent gas is contacted with at least one caustic scrubber to remove said at least one of HCl and Cl₂ from said vent gas prior to contacting with said membrane.
 19. The process according to claim 12, wherein said vent gas additionally comprises particulates, and wherein said vent gas is contacted with at least one filter to remove said particulates from said vent gas prior to contacting with said membrane.
 20. The process according to claim 12, wherein at least a portion of said retentate stream and at least a portion of said permeate stream are recycled to said casting step to provide said gaseous atmosphere.
 21. The process according to claim 20, wherein said retentate stream and said permeate stream are mixed with a mixture of N₂ and O₂ prior to being recycled to said casting step.
 22. A process for forming solidified Mg metal, said process comprising the steps of:(a) casting molten Mg metal in a gaseous atmosphere comprising SF₆ and CO₂ ; (b) cooling said molten Mg metal to form a solidified Mg metal; (c) obtaining a vent gas comprising SF₆ and CO₂ from said casting step; (d) contacting said vent gas with a first membrane in a first membrane separation unit at conditions effective to obtain a first retentate stream rich in SF₆ and a first permeate stream rich in CO₂ ; (e) contacting said first permeate stream with a second membrane in a second membrane separation unit at conditions effective to obtain a second retentate stream depleted in CO₂ and a second permeate stream rich in CO₂ ; and (f) contacting said first retentate stream with a third membrane in a third membrane separation unit at conditions effective to obtain a third retentate stream rich in SF₆ and a third permeate stream depleted in SF₆.
 23. The process according to claim 22, wherein at least a portion of said second permeate stream and at least a portion of said third retentate stream are recycled to said casting step to provide said gaseous atmosphere.
 24. The process according to claim 22, wherein said second permeate stream and said third retentate stream are mixed with a mixture of N₂ and O₂ prior to being recycled to said casting step. 